from wikipedia.org:
"... The Bosman ruling is one that allows professional football players in the European Union to move freely to another club at the end of their term of contract with their present team.
Jean-Marc Bosman was a player in the 2nd division of the Belgian football league, whose contract had expired in 1990. He wanted to change teams and move to Dunkerque, a French team. However, Dunkerque didn't offer his Belgian club RFC Liège enough of a transfer fee, so Liège refused to let him go.
In the meantime, Bosman's wages were reduced as he was no longer a first-team player. He took his case to the European Court of Justice in Luxembourg and sued for restraint of trade. After a tough legal battle he won his case, and on December 15, 1995 the court gave him and all other EU football players the right to a free transfer at the end of their contracts, with the provision that they were transferring from one EU Federation to another. Previous to that, professional clubs were able to prevent players from joining another club even if their contracts had expired. The Bosman ruling can be compared to the Curt Flood case in baseball, which led to the elimination of the reserve clause and the advent of free agency.
The Bosman ruling also prohibited domestic football leagues in EU member states from imposing quotas on foreign players to the extent that they discriminated against nationals of EU states. At that time, many leagues placed quotas restricting the number of non-nationals allowed on member teams. After the ruling, leagues could still impose quotas, but they could only be used to restrict the number of non-EU players on each team.
On Thursday 21 April 2005 UEFA's 52 member federations unanimously approved a rule designed to increase the number of locally trained players. The measure is an attempt to reverse some of the effects of the Bosman ruling. UEFA's chief executive Lars-Christer Olsson was reported by CNN to have said that some of the major clubs in Europe like Chelsea and Barcelona were not happy with this rule, and that he didn't rule out the possibility of a court challenge."
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